The present invention relates to systems and methods used in the transmission of television signals, in particular high definition and/or digital video signals.
There are perhaps only a few inventions of the past century that are more significant than broadcast television. It played and continues to play an important role in capturing historical events, shaping the mores of modern society and making the world a smaller place.
Despite the role it plays in today's society, current television systems primarily reside and depend on an analog platform, e.g., end user television sets, microwave transmitters and receivers that send and receive broadcasts, etc. In fact, the broadcast television network primarily relies on analog technology to get signals to users as it did over fifty years ago when the television set was first invented.
During this time period, however, the development of digital technology has spawned a new information age. Recognizing the present and future import of digital technology, various players within and without the television industry, including governmental agencies, equipment and component manufacturers, broadcasts, cable providers, etc., came together to formulate a new standard for the provision of digital television as a service. This effort resulted in what is colloquially referred to as high definition television (HDTV).
In 1996, the U.S. Federal Communications Commission (FCC) formally adopted a standard for HDTV. To date, however, HDTV services have not seen widespread deployment and use, neither in the U.S. nor internationally. Several economic and business factors have affected the deployment of HDTV. One such factor has been the availability of end user devices, such as HDTV sets. Until recently, HDTV sets were not readily available at a price within the reach of most individuals. Furthermore, even though HDTV sets are making their way into the marketplace, an additional obstacle to widespread deployment and use is and will be upgrading the analog broadcast network to a digital network. The capital investment or costs associated with replacing or upgrading the analog broadcast network with a digital network are tremendous. These costs have contributed to delaying the availability of HDTV services. In fact, the widespread deployment of and ultimate switch to HDTV will continue to be delayed, especially at small to mid-market affiliates and independent stations, until a cost-effective solution to completing a digital upgrade is available. For example, the Public Broadcasting System (PBS) broadcasts its signals to many localities via multi-hop links. The costs associated with replacing the equipment in each locality is prohibitive for independent stations such as PBS. In addition, some end-users may delay purchasing HDTV sets until a majority of the programming received is in a digital format. Thus, there are advantages to not deploying HDTV services piecemeal. As such, a solution that allows for the provision of HDTV service without completely replacing the analog equipment already in the network is needed.
In addition, in 1997 the FCC mandated in ET Docket 95-18, that the 2 GHz frequency band used by U.S. broadcasters for ENG (Electronic News Gathering), defined as BAS (Broadcast Auxiliary Services) in part 74 of the FCC regulations, must be changed by a date to be determined. A final order was issued on Nov. 5, 2003 as FCC 03-280. The final order changes the 2 GHz BAS band from 7 channels with each occupying a 17 MHz bandwidth (one channel is 18 MHz) to 7 channels with each occupying a 12 MHz bandwidth.
In the past, ENG broadcasters have used FM Analog modulation for transmission of video with up to four audio signals within these seven 17 MHz wide 2 GHz channels. Analog modulation as used by the broadcasters requires the full 17 MHz of channel bandwidth. With the channel bandwidth reduced to 12 MHz, the quality of the video and audio signals transmitted may be degraded. Furthermore, adjacent channel interference will be increased. Moreover, the number of audio channels will be reduced to only two with the second audio channel being extremely susceptible to adjacent channel interference.
The changes mandated by the FCC may force most ENG operators to turn to digital modulation. This will require replacing their analog transmitting and receiving equipment with digital equipment. The estimated cost per transmitter is about $45,000 while the estimated cost per receiver is approximately $25,000. The transmitter cost is divided between two pieces of equipment; a digital transmitter (capable of both analog and digital operation) and a digital modulator that produces a COFDM (coded orthogonal frequency division multiplexing) digital signal. The costs are usually apportioned as follows: transmitter—$20,000 and modulator—$25,000. The receiver essentially consists of two parts, the receiver and the COFDM demodulator. The costs associated with the receiver are usually apportioned as follows: receiver—$13,000 and demodulator—$6,500. In addition, the receiver also includes miscellaneous equipment, which costs approximately $5,500. Such miscellaneous equipment may include for example, a block down converter, low noise amplifier and other ancillary equipment.
It has been estimated that approximately 20,000 such transmitters and 6,000 receivers will need to be replaced. The digital modulation of choice for the ENG application is COFDM with a modulation selectable format of QPSK (quadrature phase shift keying), 16-QAM (quadrature amplitude modulation) and 64-QAM. QPSK is the primary format used with a code rate FEC (forward error correction) of ½, a guard interval of ⅛ or 1/32 and an adjustable bandwidth of 8 MHz. The bandwidth can be switched between 6, 7 or 8 MHz. COFDM is a robust form of digital transmission and works well in a multi-path environment. It is particularly robust in mobile applications such as moving vehicles and helicopters. FIG. 1 shows the trade-offs of data rates for COFDM between formats, code rates, guard interval and bandwidth. COFDM is preferable because of its multipath performance. On the other hand, the overall data rate of COFDM is relatively low as compared to other modulation formats such as single carrier VSB (Vestigial Side-Band) or QAM.
A measure of data rate or information that can be transmitted through a channel or system can be expressed in terms of bits/Hz. That is:Bits/HZ=Data Rate/Bandwidth.The maximum data rate that can be transmitted using COFDM with the QPSK format is 6.03 Mb/s within an 8 MHz bandwidth ( 6/8=0.75 bits/Hz). Many users actually operate at 5.53 Mb/s (0.69 bits/Hz). Using a less rugged format of 16 QAM, a code rate of ½ and a bandwidth of 8 MHz, the maximum data rate is 12.06 Mb/s (1.5 bits/Hz). When the bandwidth of the 8 MHz COFDM signal is reduced to 6 MHz, the data rate capacity is reduced by 6/8 or ¾. The bits/Hz, however, is reduced to 0.56 bits/Hz. As such, COFDM will limit the performance of ENG systems operating within the newly prepared FCC bandwidth limitations.
ATSC (Advanced Television Systems Committee) 8-VSB modulation (generically referred to as VSB—see Table 1) is the method adopted and used today in the U.S. to transmit High Definition TV (HDTV) to the home. The ATSC VSB modulation standard defines five modulations formats for transmitting digital data at various data rates as is shown in Table 1. In one format Trellis coding (“T”) is used with an 8-VSB formatted signal, designated 8-VSBT, by selecting two data bits at a time and adding a third bit to the two bits selected to form a three-bit word. Each three-bit word is then used to generate a symbol. Without Trellis coding the signal is not as robust and requires a stronger signal level at the receiver for error correction. Using the standard ATSC 8-VSBT format for modulation, 19.39 Mb/s of data can be transmitted in a 6 MHz bandwidth (19.39/6=3.23 bits/Hz). 8-VSBT is generally regarded as being more spectral efficient, more robust, relatively inexpensive to implement and allows for the transmission of a relatively larger amount of data to be transmitted in the same bandwidth (3.23 bits/Hz for COFDM in a 6 MHz bandwidth).
TABLE 1ATSC VSB Formats Versus Data Rates1.2-VSB = 9.696 Mbps (No Trellis Coding)2.4-VSB = 19.393 Mbps (No Trellis Coding)3.8-VSBT = 19.393 Mbps (Uses Trellis Coding)4.8-VSB = 29.089 Mbps (No Trellis Coding)5.16-VSB = 38.785 Mbps (No Trellis Coding)
Of utility then are methods, apparatus and systems that offer a reduction in the capital investment associated with upgrading the broadcast network to a digital network and ultimately providing HDTV service. Of utility also are methods and apparatus that facilitate provision of electronic news gathering in accordance with the more restrictive bandwidth requirements mandated by the FCC, while delivering the level of performance to which the ENG community is accustomed.